Not applicable.
Not applicable.
The invention relates generally to optical switches, and more specifically to a micro-electro-mechanical system (xe2x80x9cMEMSxe2x80x9d) optical switch with a mirror that rotates in the major plane of the device.
The use of optical signal transmission is rapidly growing in the telecommunications (xe2x80x9ctelecomxe2x80x9d) industry. In particular, optical transmission techniques are being used for local data transfer (xe2x80x9cmetroxe2x80x9d), as well as point-to-point xe2x80x9clong-haulxe2x80x9d transmissions. Similarly, the number of optical signals, or channels, carried on an optic fiber is growing. The implementation of wave-division multiplexing (xe2x80x9cWDMxe2x80x9d) has allowed the number of channels carried on a fiber to increase from one to over 16, with further expansions planned.
Thus, the need for optical switching technology is expanding. In the case of WDM technology, an optical channel is removed from a multi-channel fiber with an optical bandpass filter, diffraction grating, or other wavelength-selective device, and routed to one of perhaps several destinations. For example, a channel on a long-haul transmission line might be routed between a local user for a period of time and then switched back onto the long-haul transmission line (xe2x80x9cre-insertedxe2x80x9d). This is known as 1xc3x972 switching because a single input is switched between one of two possible outputs. As the complexity of optical networks grows, the complexity of the desired switching matrices also grows.
Switching matrices are being developed for several different optical network applications. xe2x80x9cSmall fabricxe2x80x9d applications have been developed using 1xc3x972, 2xc3x972, and 1xc3x978 type switches. However, there is a need for xe2x80x9cmedium fabricxe2x80x9d applications that can provide 8xc3x978 up to and beyond 32xc3x9732 type switching arrays, and even for xe2x80x9clarge fabricxe2x80x9d switching arrays that can handle 1024xc3x971024 or more switching applications. The switching arrays that allow any input to be connected to any output are generally called xe2x80x9ccross-connectsxe2x80x9d, but in some applications there may be limited switching of some ports.
Unfortunately, attempting to merely scale the techniques developed for small fabric applications may not meet system requirements, such as switching speed, switching array space limitations, and power limitations. In particular, it is often desirable to upgrade an optical network to handle more traffic by adding additional channels onto the installed fiber base, and that the switching arrays be able to fit into the existing xe2x80x9cfootprintxe2x80x9d allowed for the switching matrix. In many cases, the footprint is actually a 3-dimensional restriction. Similar restrictions might apply to the available power, or allowable power dissipation.
Various techniques have been developed to address the problems arising in the development of more complicated switching arrays. Several approaches have adapted photolithographic methods developed primarily for the field of semiconductor processing to the fabrication of optical switching arrays. In one approach, MEMS techniques are used to create a very small motive device (motor), such as an electrostatic comb drive, electrostatic scratch drive, magnetic drive, thermal drive, or the like, attached to an optical switching element, typically a mirror. The mirror is usually either fabricated in the major plane of the process wafer and rotated to become perpendicular to a switchable light signal, or is fabricated perpendicular to the major plane of the wafer. In the first instance, establishing and maintaining verticality of the mirror is very important to insure that the light signal is reflected to the desired output port. In the second instance, fabricating a mirror-smooth surface on a vertical plane of the wafer can be difficult, as can be depositing a reflective metal layer on that surface.
Similar challenges arise from speed and power requirements. Generally, a higher switching speed for a given type of actuator requires greater power. MEMS devices are attractive in that their small size often results in low power consumption, but this may also limit the inertia of the optical element that can be switched within the required period. The inertia can be changed by reducing the mass of the optical element, but this may result in an optical element that is not sufficiently rigid to reliably perform the desired optical switching function.
Thus, a need exists for optical switches that rapidly change states with relatively low power requirements. It is further desirable that such switches have a small size, but yet provide an optical element that achieves low insertion loss.
A (xe2x80x9cMEMSxe2x80x9d) optical switch is formed on an SOI wafer having a silicon substrate separated from a single-crystal silicon superstrate by a thin layer of silicon dioxide. A base portion of a die cut from the wafer is attached to a pivoting member formed in a layer of single-crystal silicon with a hinge formed of the layer of single-crystal silicon. The pivoting member rotates relative to the base portion about an axis essentially perpendicular to the major surface of the die.
A mirror attached to the pivoting member is formed from the layer of single-crystal silicon and has a mirror surface congruent with a major crystalline plane. A high-quality reflective coating of gold or other metal, or a dielectric stack can be deposited on the mirror surface because it is an open surface. A latching spring holds the pivoting member, and hence mirror, in one of two switch positions.
A magnetic drive is actuated with a simple pulse that both accelerates and decelerates the pivoting member as it switches between states. The impedance of the magnetic drive can be measured to determine the position of the switch, or a separate sensing circuit can be integrated onto the MEMS die to determine switch position.
In a particular embodiment, the backside of the mirror is thinned to reduce the mass of the mirror and thus the inertia encountered when switching between states. In a further embodiment, the backside of the mirror is patterned with reinforcing ribs to maintain a rigid mirror while reducing its mass. In another embodiment, both sides of the mirror are reflective. Release of the relatively large mirror structure from the bonding layer is achieved by etching through the substrate to allow etching of the bonding layer.